The present invention relates generally to improvements in electrophotographic sensitive members for use in an image transfer type copying apparatus and it relates more particularly to an improved electrophotographic sensitive member which comprises a photoconductive insulating layer with cadmium sulfide and cadmium carbonate as its main constituents and preferably with an insulating protective layer thereon, and to a method for producing the same.
Electrophotographic sensitive members which utilize a photosensitive material of CdS.nCdCO.sub.3 (0&lt;n.ltoreq.4) as a photoconductive insulating layer and processes for their production are described in U.S. Pat. No. 3,494,789 and U.S. Pat. No. 3,589,928. The photosensitive member of the type described in these patents has the advantages that the member can be electrostatically charged to both positive and negative polarities and that the manufacture thereof is relatively simple and inexpensive whereas the disadvantages or drawbacks which this type of photosensitive member has are that with a photosensitive material of CdS.nCdCO.sub.3 alone, satisfactory photosensitivity and charging characteristics (i.e., charging capability) cannot be attained thereby requiring the addition of an acceptor impurity of copper or silver as an activator to the photosensitive material in the calcination step or in the dispersion step into the resin binder of the CdS.nCdCO.sub.3 or in the sulfide reaction step of CdCO.sub.3 in the manufacturing process, and that in the actual copying operation, memory phenomenon occurs which is the reproduction of a residual image of the previous original on a next copy image.
As an example of a process for producing a photoconductive powder of CdS.nCdCO.sub.3 which is described in U.S. Pat. No. 3,589,928, an aqueous solution of cadmium ions, carbonate ions and hydrogen sulfide, ammonium sulfide or sodium sulfide is reacted to form a precipitate and the precipitate is then calcined at a temperature of less than 400.degree. C. to obtain the powder. In this process, the precipitate itself is obtained in accordance with the following reaction formula (1) which is: EQU CdCO.sub.3 +S.sup.2- .fwdarw.CdS.nCdCO.sub.3 ( 1)
However, satisfactory photosensitivity and charging capability cannot be attained with the photoconductive material of CdS.nCdCO.sub.3 alone and for this purpose, it is necessary to add an acceptor impurity of copper as described above. This is generally accomplished by the process satisfying the reaction formula (2) in which the desired amount of copper is added in the reaction of CdCO.sub.3 +S.sup.2- of formula (1): EQU CdCO.sub.3 +Cu.sup.2+ +S.sup.2- .fwdarw.CdS(Cu).nCdCO.sub.3 ( 2)
Accordingly, copper is diffused in the cadmium sulfide to improve the sensitivity characteristic of the photoconductive material in the process described above. However, regardless of the process used to produce the photoconductive powder, the electrophotographic sensitive member which comprises a photoconductive insulating layer of CdS.nCdCO.sub.3 is greatly influenced in its spectral sensitivity depending upon the amount of copper added, and in particular, the peak of sensitivity tends to shift relatively toward the long wave length as the amount of copper added increases, and as a result, the photosensitivity to white light is improved and there is also exhibited a tendency to stabilize the charging characteristic.
The spectral sensitivity strongly influences the reproduction of an image from a color original and there arises the problem that reproducibility of an image from an original having a reddish color image becomes notably poor if the amount of copper added is relatively large in quantity. This, of course, could be solved by reducing the amount of copper so as to establish a spectral sensitivity in the region close to relative luminous efficiency. But, a difficulty is encountered in uniformly adding a minute amount of copper in cadmium sulfide particles and as a consequence, the electrophotographic sensitive member thus manufactured would tend to exhibit manufacturing instability particularly in dark decay speed, charging capability and spectral sensitivity due to non-uniform diffusion of copper in the cadmium sulfide.
In addition to the aforedescribed drawbacks, another drawback is experienced by the CdS.nCdCO.sub.3 resin binder type photosensitive member in which the memory phenomenon is exhibited. This, as described above, is the phenomenon in which a residual image of a previously copied original is reproduced on a subsequently copied image and the cause thereof is assumed to originate from differences in the sensitivities of the image area and the non-image area between previous and next consecutive copyings. Thus, a clear and high contrast image cannot be obtained unless these drawbacks are solved.
The CdS.nCdCO.sub.3 photoconductive layer has still further drawbacks in the surface hardness, smoothness and abrasiveness of the layer itself and when the same is employed, for example, in a powder image transfer type copying apparatus, the surface thereof will be damaged and worn out by physical contact with developer and cleaning means. Such drawbacks are particularly noticeable when an elastic blade having an edge contacting the photoconductive layer is used as the cleaning means since some residual developer tends to become buried in the photoconductive layer rather than being scraped off therefrom causing what is known as a filming phenomenon. This will cut the life of photoconductive layer making it unsuitable for long use.